Hi Stanley,
As I know you are a high school student, well have you ever think of one day when you go to the university several years later, the world changed, there may be no MCS51 core cpu anymore??!! Don't know! And have you ever encountered other MCUs such as Motorola, ZiLog, AVR, PIC, ARM ..... ? The 51 is now the most popular CPU in the MainLand China and also the most popular teaching CPU in Mainland's university, anyway sticking on one CPU is not a wise choice, you should learn and should get familiar with C and other high level language such as JAVA so you can go across the platform! Personally, I like all ASM, I started my programming with a TI58 calculator that the programming OP codes were just simple mechine codes. Then I worked with other CPUs in different computer systems such as the Z80 in the TRS80, the TI's TMS9980 in the TI's board, the 6502 in the APPLE ][, the x86 in the PC, the 68705, the dsp, then the 51, and now switching to ARM. I enjoy the fun in programming in low level language because you can control everythings in there. Anyway if you are really doing big project, the low level is not fun anymore especially when the project due date is comming and your project leader pushing you to output your part.
So for me, if I am doing something in my spare time for fun, I love ASM, but for work, I need a language that can help me to get my work done as easy and quick as possible. If a language that has this command: "Read the users' requirements and generate functional units to solve the problem and get the work done!" , whow that is too good!
Finally a few words more, I suggest you should sharpen your maths skill as maths is the foundation of science and engineering. Say for your favour Robotics, you may encountered lots of maths topics in University such as Fuzzy logic controller, Neural network control, task secheduling, AI ....etc.
Hope you success in your study and your projects.
Best 73 from Li.